Bill,
"Stack loading" isn't new. When we reloaded lead, the first shot was 7 1/2's or 6's, followed by 5's or 4's. 7 1/2's will do a number on ducks up close, even "big" ducks.
The problem with it is that you have to remember to re-do the loads after the birds come in - and in a pump or auto, remembering load sequence, etc. And, it's either keep the shells in separate pockets (and which one has 4's? right or left?), separate boxes in the boat, or...
If I shoot Hevi-Steel, I shoot the 3" #4's. If I'm shooting at all well, it's a stone killer load through my Citori. When I shoot normal steel loads, it's 3" #2's. I've tried some 6's or 4's in the timber for a first shot, but just found that keeping it simple is better...one less thing to worry about.
I buy #6 or #4 Hevi-Shot for my wife to shoot. Is it better than steel? Dunno, but what I do know is that it gives her more confidence, and for the few times that she goes with me in a season (3 or 4), I want her to put the gun up knowing the bird is dead. I watched her pass shoot the last wood duck of a three person limit last year at treetops level with 2 3/4" Hevi #4's and an imp. cylinder choke. We killed ducks from over the decoys at 15 feet to treetops high, and watching her shoot was just a pleasure.
I have to get a new load for the 10-gauge this year. Up until last year I shot #1's; it was the single best patterning load for that gun, regardless of distance, and I tried everything from #3 to BBB. From big Canadas in Manitoba, to puddle ducks and divers down here in Arkansas, it was the right load for the gun and would kill ducks and snow geese stone dead at or beyond 60 yards if required. Winchester stopped making the load, however, so I have to find out what #2's will run the best through it.